This invention relates to a linear bearing for use in moving an object along a rectilinear path with a minimum of friction. More specifically the invention pertains to a linear bearing of the type incorporating groups of recirculating antifriction balls and wherein the direction of action of the load transmitted can be either downward, upward, or opposite lateral directions. The linear bearing according to the invention finds applications in machine tools, machining centers, and a variety of other pieces of machinery or equipment wherein one part is to be moved linearly relative to another.
Japanese Patent Laying-Open No. 55-72912, filed by the assignee of the instant application, discloses a linear bearing of the type under consideration. It has a bearing body of inverted-U-shaped cross section mounted astride a guide rail via groups of antifriction balls for rolling movement thereon. The bearing body has ball raceways of arcuate cross section formed longitudinally on its inner surfaces, and ball return passageways extending longitudinally therethrough. Screwed to the opposite ends of the bearing body, a pair of end covers have each formed therein curved grooves each intercommunicating one raceway and one adjacent return passageway in the bearing body. Thus, as the linear bearing travels in either direction along the guide rail, the antifriction balls recirculate along the raceways and through the return passageways.
The above prior art linear bearing can largely well accomplish the purposes for which it is intended. It has, however, proved to have certain problems in the fabrication of some of its components.
One of the problems concerns the bearing body with its ball raceways and ball return passageways. The machine-shaping of these raceways and passageways, as by grinding and broaching, has been a difficult task because of the complex shape of the bearing body. Consequently the raceways and passageways have not necessarily been shaped to close tolerances. Too much time has also been expended for the job, adding to the manufacturing cost of the linear bearing.
There is another reason why the creation of the ball raceways and return passageways in the bearing body is undesirable. The ball raceways require hardening as by induction heating and subsequent quenching. Again because of the complex shape of the bearing body, however, a difficulty has been encountered in hardening only its desired parts. Thus the bearing body has been easy to suffer strain, or its entire desired parts have not necessarily been hardened to a required degree.
Another problem has arisen in conjunction with the pair of end covers fastened to the bearing body to provide the curved grooves intercommunicating the ball raceways and return passageways in the bearing body. The end covers in the noted prior art linear bearing have been each integrally molded of plastics material. The one-piece molding of each end cover is objectionable by reason of its complex shape. Having approximately the same shape as the cross section of the bearing body, the end covers have formed therein not only the curved grooves serving as parts of the closed ball paths but also annular depressions and other recesses. Much difficulties have been involved in the production of molds for such complex shapes and in the control of the heat and pressure conditions for the molding of the end covers therein. Some errors in the dimensions of the molded end covers have therefore been almost unavoidable, resulting in the hindrance of the rolling of the antifriction balls in the curved grooves in the end covers. The fabrication of such unitary end covers have also been very time-consuming and so added considerably to the cost of the linear bearing.
A further problem with the prior art linear bearing concerns a unitary cage rollably holding the antifriction balls on the raceways on the bearing body. Generally horseshoe-shaped in cross section to conform to the interior contours of the bearing body, the cage has slots formed longitudinally therein allow the balls to make rolling engagement with raceways on the guide rail. The cage is made from sheet steel by means of a press. Here again the complex shape of the cage has rendered it difficult to create the slots in exact positions thereon, with the consequent impediment of the travel of the balls in proper rolling engagement with the raceways on the bearing body and on the guide rail.